Add a property for cow_http_hd:parse_accept/1

Also fixes a few bugs uncovered by the property, and also allow
empty Accept values as per RFC7231.
Add a property for cow_http_hd:parse_accept_charset/1
Add a property for cow_http_hd:parse_accept_encoding/1
Add a property for cow_http_hd:parse_accept_language/1
Fix a shadow var warning in a property
Add a property for cow_http_hd:parse_connection/1
Add property for cow_http_hd:parse_expect/1
Add property for cow_http_hd:parse_max_forwards/1
Remove an unused test function
Add a property for cow_http_hd:parse_transfer_encoding/1
Add cow_http_hd:parse_if_modified_since/1

From RFC7231 and RFC7232.

The new code is faster than the one in Cowboy: 2 times faster
for fixdate format, 2.5 times faster for rfc850 format and 3+
times faster for asctime format.

Unlike previous code, there is no parsing speed difference
between the 3 different date formats.
Add cow_http_hd:parse_if_unmodified_since/1